1. Field of the Invention
This invention relates to digital halftoning for printing. In particular this invention relates to a method for producing halftone printing plates containing microscopic ink repelling areas.
2. Description of the Prior Art
Printing can be done using different printing processes. The most important processes currently used are offset, gravure and flexography. Each of these processes has its advantages and disadvantages, and these processes compete with each other in price, quality and flexibility. Currently, offset is the most important process, certainly when the printing substrate is paper.
The offset process produces a printed result on a substrate by bringing over ink selectively from an ink source to an offset plate, then to an offset blanket and finally onto the substrate. Whether the final substrate will be inked (printed) or not in a particular location is determined by whether or not a corresponding zone on the printing plate will be inked.
Offset plates are flat plates which, when ready for printing, contain both ink repelling and ink receptive zones. The ink receptive zones receive ink and transfer the ink to an offset blanket, which in turn transfers the ink onto a substrate which may be paper, carton board, or other media. In traditional offset printing, the ink repelling zones accept water, and once humid, do not accept ink, so that the corresponding areas on the substrate are left blank. Waterless offset printing methods also are known in the prior art.
Whether traditional or waterless offset is used, the location of the ink receptive zones typically is determined by digital artwork, which may include linework (also called line art), continuous-tone, xe2x80x9ccontone,xe2x80x9d (xe2x80x9cCTxe2x80x9d) images, or a combination, and the way the artwork is screened. Screening is the process of creating the illusion of a continuous-tone image on a device which can only reproduce two output valuesxe2x80x94ink or no ink in the case of an offset printing press. The continuous-tone illusion is created by the placement of small printing elements, known as halftone dots. When screening is done through digital operations, it is sometimes called digital halftoning. Thus, xe2x80x9cdigital halftoningxe2x80x9d shall refer to such digital screening herein.
The way the digital artwork determines the plate is straightforward in areas where the digital artwork is blank (no ink needed, 0% fill) or fully filled (100% fill). Where it is blank, the plate will be fully ink repelling and where it is fully filled, the plate will normally be fully ink receptive. In all other cases, screening typically is used to obtain the desired appearance on the final printed result.
In some of the prior art, traditionally, the screening process places halftone dots centered on a square grid determined by a fixed angle and a fixed periodicity. The periodicity sometimes is expressed as the ruling, which is frequently expressed as a number of dots per inch (dpi), and sometimes also as the number of lines per inch (lpi) since the dots are usually arranged in lines. Density variations are achieved by changing the size of the halftone dots while the number of halftone dots in a given area is the same for high densities as for low densities. For example, 10% and 50% density regions have the same number of halftone dots but the area of a 50% dot is 5 times larger than the area of a 10% dot. This kind of screen is known as the xe2x80x9cclassical screen.xe2x80x9d The technology of classical screens is very popular and covers currently more than 95% of the printing business. Variation of the size of the halftone dots in digital halftoning for classical screens is done by varying the number of black pixels making the halftone dot.
FIG. 2 shows a gradation of a classical screen 201 and is thus labeled xe2x80x9cPrior Art.xe2x80x9d
In classical screening, whether done digitally or not, the angles and periodicity of the square grid of dots are determined by technical considerations. For example, color printing involves printing in several colorants (inks), and several plates are prepared, one for each ink. These are called color separations. Printing is achieved by overlaying the different colors. Different angles are used for each separation to avoid moirxc3xa9 when overlaying the different printing colors. Rulings are determined by the process parameters such as substrate quality, ink viscosity, press characteristics and so on. Generally, one tries to use a ruling as high as possible to achieve the best image sharpness, taking care, however, not to exaggerate the ruling in order to avoid technical problems on the press.
Producing good quality printed material with an offset process is not an easy task. As is known in the printing industry, there are many problems which affect the quality of the final printed result, some even making printing impossible. The following is an incomplete list of some of the important problems.
substrate feeding problems causing press stops, bad quality or excessive waste of substrate;
density variations (e.g., too dark, too light, moirxc3xa9 patterns, etc.);
drying problems (e.g., ink dries too early or too late);
ink water balance (e.g., too much water, not enough water, etc.); and
inking problems (e.g., too much ink, not enough ink, ink on wrong places, etc.).
All such problems have in common that they diminish the quality of the produced printed material or the productivity of the press or increase the waste of substrate.
One of the choices one has to make for an offset printing is selecting the ink. On an even, high quality substrate, one can choose a strong, concentrated ink, with a brilliant aspect. By nature, strong inks typically are very sticky (high tack) and pull on the substrate during printing. When used on lower quality substrates, which are weaker or may have a rough surface, strong inks tend to pull particles away from the surface of the paper. Moreover, forces which pull on the substrate itself can cause shifts of the substrate in the press, and can lead to register problems or dot slur and even dot doubling. In an extreme case, the substrate can remain sticking on the press blanket and cause a paper jam, which unavoidably causes a press stop. On a web press, the web can beat due to the substrate which first sticks to the blanket, then gets pulled off by the web. This causes a dirty printed result. The operator typically solves this problem by making the inks xe2x80x9csofterxe2x80x9d, through the use of additions to the ink or the water, or by choosing softer inks supplied by ink manufacturers. As a result, dot gain can increase and the final result will be less brilliant. Moreover, the press will typically be run with too much water. The printing results will be pale. There is also an increased risk that moisture may enter the ink rolling system. When this happens, ink and water make a mixture which introduces quality problems and causes the ink rolling system to require more frequent cleaning.
The printing industry has been searching for solutions to these problems for a long time. Over the years, printing presses have improved in quality and degree of control, new ink types have been developed, and substrate manufacturers have delivered higher quality substrates and products of more consistent quality.
An important consideration in printing is the printing plate itself. Depending on the nature of the image on the printing plate, the above mentioned problems exist to a greater or lesser extent. There is thus an economic incentive to define imaging methods that produce printing plates which guarantee less troublesome printing.
As would be clear to those of ordinary skill in the art, a film often is used to make an offset plate, and whatever characteristics are required in the offset plate, corresponding characteristics also would be required in the film, the correspondence depending on whether the film is positive or negative. Thus, it is to be understood that the word xe2x80x9coffset platexe2x80x9d when used hereunder means the offset plate and/or the corresponding film for making the offset plate for the case that such a film is used.
The term xe2x80x9cexposurexe2x80x9d here is used to describe the physical process of making a plate. When a film is involved, the term exposure is the process of exposing a photosensitive plate with radiation through the film. For direct computer-to-plate processes, the term exposure is the output of the computer data to the plate, for example using a raster scanning laser or other computer-to-plate methods.
The most common method of improving printability through the nature of the image on the printing plate is by reducing the ruling of halftones. Lower rulings make bigger dots. As a result, process variations have a smaller influence on the density of the final printed result. Popular rulings are 133, 150 and 175 lines per inch (lpi) for high quality presses on quality paper, 80, 100 or 120 lpi for work on difficult substrates such as plastic or highly absorbent paper, and even lower lpi values, for example, for newspaper printing, where substrates are cheap and presses must run at very high speeds. An important disadvantage of lower rulings is loss of sharpness.
Another way of influencing printability through the properties of the printing plates is described in published French patent, publication number 2 660 245, to Nouel, publication date Oct. 4, 1991, entitled Plaques ou clichxc3xa9s destinxc3xa9s à l"" impression, procxc3xa9dxc3xa9 pour leur prxc3xa9paration, films ou caractxc3xa8res utiles à leur prxc3xa9paration, leur utilization en imprimerie, (translated to xe2x80x9cPlates for a printing press, methods for their preparation, films or characters used in their preparation, their use in printingxe2x80x9d), incorporated herein by reference, and in published international (PCT) patent application number WO 96/02868, also to Nouel, publication date Feb. 1, 1996, entitled Use of frequency-modulated screening for lightening offset printing surfaces, and incorporated herein by reference. These patents (hereinafter xe2x80x9cExisting Nouel Patentsxe2x80x9d) state that better printing results are easier to achieve on the press when the printing surfaces which receive ink contain small ink repelling parts. These ink repelling parts are small because it is intended that during printing, the areas will be filled-in with ink. This is in contrast with the normal ink repelling zones in an offset plate, which, after printing, normally would be blank. In this document, these small ink repelling parts shall also sometimes be called perforations.
The Existing Nouel Patents describe a method to include perforations in an offset plate through the following steps, which involve traditional production of offset plates via exposing light sensitive plates through a corresponding film:
1. Take a normal offset plate, in this case a positive offset plate. The term positive offset is used for a light sensitive offset plate (exposed using a corresponding film) which will be ink repelling where it is exposed to light.
2. Expose the offset plate through the positive corresponding film containing the image.
3. Expose this same plate a second time through a second film, which is completely opaque, except for a number of white spots, typically comprising approximately 10% of the surface. Measured by a densitometer, this film will have a density of around 0.9 D on average. The sizes of the perforations typically are in the order of 25 microns square.
4. As a result of the second exposure, the parts of the image which were not ink repelling after the first exposure and which were exposed to the light in the second exposure because they were under one of the white spots in the second film, will now also become ink repelling.
Many variants are possible:
One can add perforations with an analogous process for negative plates.
The number and the nature of the perforations can vary. Typically, more perforations are added for newsprint, i.e., newspaper paper types (perforations are added to up to 20% of the surface) than for high quality paper types (typically below 8% of the surface). The perforations are around 25 by 25 microns square in size, but these can be made smaller or larger depending on the nature of the printing process, the press, the printing inks, paper stocks, etc.
Distribution of the perforations can follow a classical screen, or more typically, a stochastic screen. Stochastic screening, also called random or frequency modulated (FM) screening, keeps dot size constant, but varies the number of dots per unit area in order to simulate continuous-tone changes in gray-scale. This is different from a classical screen, in which halftone dots are placed on a regular grid at some angle and in which the size of the halftone dot is varied to simulate continuous-tone changes.
The method produces the following advantages in printing:
The use of the perforations allows higher quality printing than conventional methods with a more consistent ink and water deposit, especially in dark areas;
Printing is made easier where large areas of 100% ink are in proximity to areas using less ink, for example less than 10%, due to the implicit regulation of the water deposit;
Stronger inks with more tack can be used for more paper stocks;
Lower ink consumption is obtained;
Faster press set-up is achieved because fewer registration problems occur, and because the ink/water balance is less critical than in conventional offset printing;
Ink sticks less on the press blankets, and therefore, washing the blankets is made easier; and
The undesirable effects of tacky inks are diminished when such inks are used in waterless offset printing. Waterless offset printing typically requires inks that are extremely tacky, otherwise, the plate does not take the ink in the correct way. As a result, low quality paper is difficult to print on a waterless offset press. Perforations make it possible to produce acceptable printed results on low quality paper with waterless printing methods.
Most of these advantages are typically very important on bad or old presses, or when substrates or inks have low quality. In other words, perforations allow for higher quality with cheaper material.
The main problem of the perforations method in the prior art is in the way the perforations are added:
Additional steps in the pre-press flow are needed. At the minimum, an additional exposure is required, for example involving producing an additional film. Any additional steps take time and require: additional material, skilled people and, when putting two physical image carriers on top of each other is involved, a good vacuum in the copy frame;
The additional film is delicate and can diminish the final plate quality;
When positive platesxe2x80x94plates on which the parts exposed to light become ink repellingxe2x80x94are produced by a direct computer-to-plate process, the plate has to be exposed twice before processing. This double exposure makes automation of the process difficult. Typically, computer-to-plate imagesetters are automated to take a plate from a cassette, expose it once and feed it directly to an on-line processor;
When negative plates, plates on which the parts exposed to light become ink receptive, are produced by a direct computer-to-plate process, there is no simple method to integrate the perforations into the plates. One possibility would be to fabricate plates which have perforations on certain spots no matter whether they are exposed to light or not; and
Since perforations are typically added through a manual process of putting two physical image carriers on top of each other, there is little control over the placement of the perforations.
Thus there is a need in the art for an offset plate with perforations that is produced with one exposure and by a method that enables overcoming many of the disadvantages mentioned above.
Thus, one object of the present invention is an offset plate with perforations, each part of the offset plate produced with a single exposure. It is a further object of the invention to provide a method and apparatus for producing halftones with perforations. It is also an object of the present invention to provide a method and apparatus for producing digital halftones with perforations. It is a further object of this invention to provide a method and apparatus for producing digital halftones with perforations, the method allowing control of the placement, size, shape and distribution of the perforations.
These and other objects are provided for in the present invention. One aspect of the present invention is an offset plate with perforations, each part of the offset plate produced with a single exposure. Another aspect is a method and apparatus for producing digital halftones with perforations. Another aspects of the invention is a method and apparatus for producing digital halftones with perforations, the method allowing control of the placement, size, shape and distribution of the perforations.
Another aspect of the invention is a method for producing digital halftone screen masters with perforations. Another aspect is using such screen masters to screen continuous-tone (CT) and linework originals to generate screened versions with perforated screening, the perforations inherent to the master screens. When such screening are used in an imagesetter, any films coming out of the imagesetter have perforations already included. Thus, exposing the plates need not take longer than if a classical screen were used. When such screening is used with a computer-to-plate system, the plates come out ready for the press, without requiring an additional step to add the perforations.
One embodiment disclosed includes a method for producing a supercell threshold array which includes perforations. Another aspect is using a perforated supercellcell threshold array for screening in the same manner as any supercell threshold array. The preferred embodiment of the method for generating the perforated threshold array starts with an original non-perforated threshold array. Perforations are added to this original threshold array. In several alternate aspects, alternate methods are disclosed for adding the perforations to the original. One such method includes generating a stochastic binary pattern of perforations using a threshold array for stochastic screening, the stochastic threshold array having the same mathematical properties as the original threshold array.
Another aspect of the invention is a method for producing, instead of a perforated supercell threshold array for screening, a set of bitmaps for screening using a bitmap lookup table screening method, the bitmaps including perforations. Thus another aspect of the invention is screening using such perforated bitmaps. Screening with such perforated bitmaps provides a degree of control of the perforations.
Another aspect of the invention is a method for adding perforations to pre-screened (bi-level) originals, the adding carried out as a computer implemented pre-processing step. The preprocessing step automatically erases pixels in the existing bitmap to produce perforations, and is preferably implemented as a digital filter.
Yet another aspect of the invention includes starting with a page description, for example, a page description in the PostScript(copyright) language, the page description in one embodiment being of the final plate, or in another embodiment of source files which then may be used to compose the image to make the final plate. Perforations are added to the page description as a bitmap perforating layer on top of all the graphic elements in the page description.
It should be clear that although some of the descriptions will be for producing a single plate, the invention is applicable to producing either a single plate for monochrome printing, or a set of plates for color printing.
Other aspects of the invention will become clear from the detailed description and from the claims.